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PERTH, Australia - Australia will deploy a modified Boeing 737 to act as a flying air traffic controller over the Indian Ocean to prevent a mid-air collision among the aircraft searching for the Malaysia Airlines jetliner that went missing over three weeks ago, an official said Tuesday.

The E-7A Wedgetail equipped with advanced radar will be deployed "in the near future" to monitor the crowded skies over the remote search zone, former Australian defense chief Angus Houston, who heads the joint agency coordinating the multinational search effort.

The three-week hunt for Flight 370 has turned up no sign of the Boeing 777, which vanished March 8 with 239 people on board bound for Beijing from Kuala Lumpur.

The search zone area has evolved as experts analyzed Flight 370's limited radar and satellite data, moving from the seas off Vietnam to the waters west of Malaysia and Indonesia, and then to several areas west of Australia. The current search zone is a remote 254,000 square kilometer (98,000 square mile) that is a roughly 2 1/2 hour flight from Perth.

Under normal circumstances, ground-based air traffic controllers use radar and other equipment to keep track of all aircraft in their area of reach, and act as traffic policemen to keep planes at different altitudes and distances from each other. This enforced separation - vertical and horizontal - prevents mid-air collision. But the planes searching for Flight 370 are operating over a remote patch of ocean that is hundreds of kilometers (miles) from any air traffic controller.

On Tuesday, 11 planes and nine ships were focusing on less than half of the search zone, some 120,000 square kilometers (46,000 square miles) of ocean west of Perth, with poor weather and low visibility forecast, according to the new Joint Agency Coordination Center, which will oversee communication with international agencies involved in the search. A map from the center showed that the search area was about 2,000 kilometers (1,200 miles) west of Perth.

The arrival of the E-7A "will assist us with de-conflicting the airspace in the search area," Houston told reporters. He did not specify when the plane would be deployed.

Rob Shearer, captain of the Royal New Zealand Air Force's P-3 Orion, on Monday warned his crew to stay alert for the growing number of planes and ships crisscrossing the area. Some of the search aircraft have been dropping as low as 200 feet (60 meters) above the water - and occasionally dipping even lower for brief periods.

"An important note to mention to all of you, there's a lot of surface craft out there now, so we need to know and have eyes on everything before we go below 1,000 (feet)" Shearer told his crew before they headed out to the search zone.

Although it has been slow, difficult and frustrating so far, the search for the missing Malaysia Airlines jet is nowhere near the point of being scaled back, Australian Prime Minister Tony Abbott said Monday.

He said that search teams are "well, well short" of any point where they would scale back the hunt. In fact, he said the intensity and magnitude of operations "is increasing, not decreasing."

"I'm certainly not putting a time limit on it," Abbott said from at RAAF Pearce, the Perth military base coordinating the operation. "We can keep searching for quite some time to come."

"We owe it to the families, we owe it to everyone that travels by air. We owe it to the anxious governments of the countries who had people on that aircraft. We owe it to the wider world which has been transfixed by this mystery for three weeks now," he said.

"If this mystery is solvable, we will solve it," Abbott said.

Malaysia has been criticized for its handling of the search, particularly its communications to the media and families of the passengers. In something likely to fuel those concerns, the government changed its account of the final voice transmission from the cockpit.

In a statement late Monday, it said the final words received by ground controllers at 1:19 a.m. on March 8 were "Good night Malaysian three-seven-zero." Earlier the government said the final words were "All right, good night."

The statement didn't explain or address the discrepancy. The statement also said investigators were still trying to determine whether the pilot or co-pilot spoke the words.

Items recovered so far were discovered to be flotsam unrelated to the Malaysian plane. Several orange-colored objects spotted by plane Sunday turned out to be fishing equipment.